Polytetrafluoroethylene-silicone rubber extrusion composition



United States Patent 3,325,434 POLYTETRAFLUOROETHYLENE-SILICONE RUBBER EXTRUSION COMPOSITION Paul R. Tully, Lowell, Mass, assignor to General Electric Co., a corporation of New York No Drawing. Original application July 20, 1961, Ser. No.

125,356, now Patent No. 3,207,825, dated Sept. 21, 1965. Divided and this application Apr. 27, 1965, Ser. No. 459,501

3 Claims. (Cl. Mil-29.6)

This application is a division of Ser. No. 125,356 filed July 20, 1961, now US. Patent 3,207,825.

This invention relates to an improved extrusion composition and to useful articles such as sheets, rods, tubes, filaments, and coatings fabricated therefrom. More particularly, the invention relates to a polytetrofluoroethylene extrusion composition modified by the presence of silicone rubber and to a conductor coated with such a composition.

A polytetrafluoroethylene extrusion composition was disclosed in Llewellyn et al. Patent No. 2,685,707. In accordance with this patent, colloidal-sized particles of poly- .tetrafiuoroethylene coagulated from an aqueous colloidal suspension of the polymer are mixed with an organic lubricant having a viscosity of at least 0.45 centipoise at 25 C., the lubricant being liquid under the conditions of subsequent extrusion and present in an amount equal to from 5% to 50% based upon the combined weight of the polytetrafluoroethylene and lubricant, to obtain a dry, uniform, pressure-coalescing mixture. The mixture is then passed through a die at a temperature of 15 C. to 150 C. under compacting pressure and the resulting product is baked at a temperature above the sintering temperature of the polymer until sintered. Preferably, substantially all of the lubricant is removed from the mixture before the temperature of the polymer reaches the sintering temperature.

Polytetrafluoroethylene is at best a ditficult material to extrude. The process for extruding thin wall tubing on thin wires with the polytetrafluoroethylene extrusion composition as described above produces an article which is apt to have at least one electrical defect every few hundred feet. Since the wire is used in critical applications where it must bewfalt-free, standard manufacturing techniques provide for complete testing of wire and severing it wherever a defect occurs. This produces fault-free wire but the lengths vary, some being as short as ten feet. Rarely is a length of more than 500 feet produced. Furthermore, it is well known that the life of wires insulated with polytetrafiuoroethylene is significantly curtailed in the presence of corona discharge at high voltages.

One of the objects of the present invention is to provide a high-temperature-stable additive for polytetrafluoroethylone which will produce a finished product which has improved electrical properties and enables fault-free wire to be produced in longer average lengths.

Another object of the present invention is to provide a polytetrafiuoroethylene extrusion composition containing an additive which is cured upon heating the extruded composition.

Another object of the present invention is to provide an insulation of polytetrafiuoroethylene modified by the addition of silicone rubber which has improved corona resistance and which can withstand the degrading effect of ionic bombardments for much greater periods of time.

3,325,434 Patented June 13, 1967 Other objects of the invention will become apparent from the following specification. Briefly stated, in accordance with one of its aspects, the invention is directed to an extruded polytetrafluoroethylene composition comprising 0.6% to 12.5 by weight of cured silicone rubber, the balance comprising coagulated aqueous colloidal polytetrafluoroethylene suspension which has been sintered.

The polytetrafluoroethylene used in this invention and described in the patent to Llewellyn et al. is sold by E. I. du Pont de Nemours & Co., Wilmington, Delaware, under the designation Teflon 6. As described in Bulletin No. X-93 of the du Pont Company, Teflon 6 is capable of undergoing large plastic deformations when subjected to moderate stresses. When compounded with an organic lubricant, such as naphtha or white oil, and forced through a restricting die, the individual particles are deformed into a compact fibrous structure. The term polytetrafluoroethylene should be understood as it applies to this invention as being a number of types of polymers such as tetrafluoroethylene homopolymers, tetrafluoroethylene copolymers comprising the polymerization product of a mixture of tetrafluoroethylene and some other unsaturated organic compound such as ethylene and chlorotrifluoroethylene, and also tetrafluoroethylene polymers in which the end groups of the polymer chain are supplied by nonpolymerizable compounds such as methanol and ethanol.

In accordance with the present invention, the polytetrafluoroethylene has mixed with it from 0.6% to 12.5% by weight (of the polytetrafluoroethylene) of silicone rubber containing a curing agent. Silicone rubbers are organopolysiloxanes which are heat curable to a rubber-like consistency in the presence of certain curing agents. While silicone rubbers generally are satisfactory for use in the present invention, the more desirable silicone rubbets are those which cure at low temperatures. These are the silicone rubbers which can be cured quickly by the application of heat. It is desired to effect the cure of the silicone rubber as quickly as possible after it has been extruded upon wire so that the silicone rubber is cured during the time required to volatilize the lubricant and sinter the polytetrafluoroethylene. Other materails may be added to the extrusion composition for the purpose of color or reinforcement. Examples of such materials include silica, asbestos, talc, mica, graphite, inorganic pigments, etc.

Silicone rubbers which are particularly useful in the present invention are those described in Berridge Patent No. 2,843,555 which describes an organopolysiloxane composition convertible at temperatures ranging from about 2040 C. to the cured, solid, elastic state. Such a composition comprises (a) a linear, fluid organopolysiloxane containing terminal silicon-bonded hydroxy groups and having an average of about two organic groups per silicon atom (b) an alkyl silicate, and (c) a metallic salt of an organic carboxylic acid in which the metallic ion is selected from the class consisting of lead, tin, zirconium, antimony, iron, cadmium, barium, calcium, titanium, bismuth, and manganese. Silicone rubbers manufactored in accordance with the Berridge Patent are sold by the General Electric Company under the designation RTV which stands for Room Temperature Vulcanizing.

The following examples are given by way of illustrating the invention and are not to be taken as limiting.

Example 1 Polytetrafluoroethylene (Teflon 6C) (540 grams) was orked in a tumbling mill for 20 minutes with silicone bber (RTV 11) (20 grams) to which .20 gram of diltyl tin dilaurate (Thermolite 12) curing agent had :en added along with 98 grams of naphtha. The mix as then blended by a 20-minute roll on a rolling mill. t the end of this time the mixture had a pasty consistency 1d it was placed into a preform tube and compressed to rm a billet for extrusion.

The silicone rubber modified polytetrafluoroethylene llet was extruded by means of conventional equipment. fter extrusion, the wire was subjected to heat treatment volatilize the naphtha, cure the silicone rubber, and nter the polytetrafluoroethylene.

The silicone rubber principally serves to fill the voids 'eated as the naphtha is volatilized thus rendering a )mposition which has outstanding electrical properties.

Example 2 The procedure of Example 1 was repeated except that nly 3% grams of silicone rubber were added along with )3 gram of curing agent and 114 grams of naphtha. The esulting compound did not extrude as readily as the comound of Example 1. The final product did not have as ood electrical properties and had more defects per unit f length. Nevertheless, it was dielectrically superior to a olytetrafluoroethylene-insulated wire which contained no ilicone rubber. 7

Example 3 The procedure of Example 1 was followed except that i7 /2 grams of silicone rubber and .70 gram of curing .gent were present and 50 grams of white oil. The com- )ound extruded very readily and wire insulated with the naterial had relatively few defects. However, these good 'esults were obtained at the expense of lowering some of he other electrical properties of the wire (i.e. power actor and specific inductance capacitance).

By far the greatest advantage that the polytetrafiuorozthylene-silicone rubber composition offers is improved :lectrical characteristics of the finished wire. Production runs resulted in fewer electrical failures resulting in longer engths of fault-free wires. Laboratory corona resistance :ests prove that the average voltage failure as applied for one minute in water occurs at 4000 volts for a 0.010" wall of polytetrafluoroethylene and 6000 volts for a 0.010" wall of polytetrafiuoroethylene-silicone rubber insulation. Samples of polytetrafluoroethylene insulated wires and polytetrafiuoroethylene-silicone rubber composition insulated wires were immersed in water and a potential of 3000 volts was supplied between the conductor and the water until failure occurred. It was found that the average life expectancy of the polytetrafiuoroethylene-silicone rubber composition was twenty times greater than that of the polytetrafiuoroethylene insulation.

Radiation resistance is an important property of the high-temperature wire insulation of the present invention. The improvement brought about by the addition of silicone rubber to polytetrafiuoroethylene as opposed to straight polytetrafiuoroethylene is well illustrated in the following Table I:

TABLE I.AVERAGE DIELECTRIC STRENGTH STRAIGHT POLYTETRAFLUOROETHYLENE INSULA- TION VS. POLYTETRAFLUOROETHYLENE-SILICONE INSULATION APPLIED TO SIZE 20 AWC 1 Insulation Cracked-Complete Mechanical and Electrical Failures.

While the invention has been described with reference to certain specific embodiments thereof, it is obvious that there can be variations which fall within the true spirit of the invention. Therefore, the invention should be limited in scope only as may be necessitated by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An extruded and sintered polytetrafluoroethylene composition comprising 0.6% to 12.5% by weight of cured silicone rubber, and the balance coagulated aqueous colloidal polytetrafluoroethylene suspension.

2. An extrudable polytetrafluoroethylene composition containing a curable component comprising 0.5% to 10% by weight of uncured silicone rubber in which there is incorporated a curing agent, 12% to 20% by weight of an organic lubricant, and the balance coagulated suspension of aqueous colloidal polytetrafluoroethylene.

3. An extrudable polytetrafiuoroethylene composition containing a curable component comprising coagulated aqueous colloidal polytetrafiuoroethylene suspension, from 0.6% to 12.5% by weight of the polytetrafiuoroethylene present of uncured silicone rubber in which there is incorporated a curing agent, and 12% to 22.5% by weight of the polytetrafluoroethylene present of an organic lubricant.

References Cited UNITED STATES PATENTS 2,419,095 4/1947 Stevens 174110 2,485,691 10/1949 Bogese 18-59 2,586,357 2/1952 Llewellyn 26033.6 2,802,897 8/1957 Hurd et al. 174--110 2,906,658 9/1959 Doban 260-29.6 2,976,257 3/1961 Dawe et al 26029.6 2,990,582 7/1961 Tully et a1. 1859 3,047,421 7/1962 Taylor 260-29.6 3,062,764 11/1962 Osdal 260-29.6 3,217,083 11/1965 Gore 26029.6

OTHER REFERENCES Teflon, Du Pont Information Bulletin, Wilmington, Del., February 1956, pp. 43-47.

MURRAY TILLMAN, Primary Examiner.

J. L. WHITE, Assistant Examiner. 

3. AN EXTRUDABLE POLYTETRAFLUOROETHYLENE COMPOSITION CONTAINING A CURABLE COMPONENT COMPRISING COAGULATED AQUEOUS COLLOIDAL POLYTETRAFLUOROETHYLENE SUSPENSION, FROM 0.6% TO 12.5% BY WEIGHT OF THE POLYTETRAFLUOROETHYLENE PRESENT OF UNCURED SILICONE RUBBER IN WHICH THERE IS INCORPORATED A CURING AGENT, AND 12% TO 22.5% BY WEIGHT OF THE POLYTETRAFLUOROETHYLENE PRESENT OF AN ORGANIC LUBRICANT. 